1. Field of the Invention
This invention relates broadly to hydrocarbon exploration and production. More particularly, this invention relates to apparatus and methods for measuring formation characteristics such as resistivity and permittivity.
2. State of the Art
In the hydrocarbon exploration and production industry, it is of practical importance to be able to detect the formation properties such as resistivity and permittivity surrounding a wellbore. Two classes of methods are generally employed for such purpose. A first class of methods, as represented by U.S. Pat. No. 2,582,314 to Doll which is hereby incorporated by reference herein in its entirety, utilizes magnetic dipoles to excite electromagnetic wavefields in the formation. A second class of methods, as represented by U.S. Pat. No. 2,712,627 to Doll, and U.S. Pat. No. 4,567,759 to Ekstrom, et al. which are both hereby incorporated by reference herein in their entireties, utilize electrodes to excite electromagnetic wavefields in the formations. In both cases, the electromagnetic wavefields permit the measurements of the formation resistivity.
It will be appreciated by those skilled in the art that the methods utilizing electrodes for measurement of formation resistivity rely on direct electric current conduction between the tool and the formation. As a result, they are only applicable in boreholes drilled with conductive muds; i.e., generally water b ase muds. However, it is desirable to have techniques which enable electrode-type methods to operate in both conductive and non-conductive (i.e., oil base) muds.
In a recent U.S. Patent Application Publication 2002/0166699 to Evans, a measurement-while-drilling (MWD) apparatus is disclosed which purports to measure formation resistivity in the presence of oil-base muds. The Evans device utilizes focusing and defocusing as taught in U.S. Pat. No. 6,348,796 to Evans et al., via the use of measurement electrodes, focusing electrodes, and a guard electrode on a pad, and a diffuse return electrode on the tool, and attempts to determine resistivity of the formation across the capacitive coupling of the non-conductive mud by measuring current at a frequency of 1 MHz. The idea proposed by Evans has many drawbacks. First, the electrode configuration of Evans is unlikely to work efficiently at the proposed frequency due to the skin effect. In other words, Evans cannot generate strong enough electromagnetic wavefields in the formation to permit a realistic measurement of the formation. Second, the focusing and defocusing scheme of Evans is undesirable in that it requires a larger measuring electrode that reduces image resolution. Over-focusing in order to overcome image resolution problems may lead to negative measured currents (and hence negative apparent resitivities) depending on the amount of standoff and the contrast between the mud resitivity and formation resistivity. It may also lead to a large squeeze effect resulting in determinations which exaggerate the thickness of conductive beds and which miss thin resistive layers. In addition, over-focusing may make the measurements more prone to borehole rugosity effects and irregular motion of the tool, particularly for larger standoff conditions.
As disclosed in U.S. Pat. No. 6,191,588 to Chen which is hereby incorporated by reference herein in its entirety, an alternate method of obtaining formation resistivity in the presence of non-conductive muds is to excite a large voltage drop within the wellbore. Since the formation is in parallel to the mud layer and is generally more conductive than the mud, the voltage variations within the borehole are more sensitive to the formation resistivity. As a result, one can obtain the formation resistivity by measuring the voltage distribution within the borehole. While this technique provides good results, it is limited to operation in oil base muds. In addition, it does not work optimally in very resistive formations, especially when the mud resistivity is not much higher than the formation resistivity.